Heretofore, white fillers such as silica, magnesium carbonate and the like were exceptional to be compounded with a rubber composition for tires because they have problems in the tensile strength, modulus of elasticity and rebound resilience of the vulcanizate as compared with carbon black for the rubber reinforcement.
On the contrary, Japanese Patent Application Publication No. 40-20,262 discloses that the slipping resistance is improved in a tire comprising a tread made by compounding silica with a rubber composition containing butadiene rubber, oil and carbon. However, it will be expected that the wear resistance is poor because the modulus of elasticity is low.
Furthermore, Japanese Patent Application Publication No. 38-26,765 proposes a method wherein rubber having a higher modulus of elasticity is obtained by mixing silica sol with a rubber latex and spray-drying them as compared with a case of the usual kneading method.
Even in this method, however, it is at the present condition that the effect is not equal to the reinforcing effect of the carbon black.
And also, a tire tread for winter season tire having an improved slipping resistance by using a silane compound containing silica and sulfur atom is proposed in Japanese Patent laid open No. 50-88,150. However, it is required to use a large amount of the silane compound for obtaining preferable tread properties.
Moreover, Japanese Patent Application Publication No. 49-36,957 proposes a method wherein a lithium-terminated polymer obtained by using an organic lithium compound as a catalyst for the purpose of improving the processability is reacted with silicon tetrahalide, trichloromethyl silane or the like to produce a branched polymer centering the silane compound. However, the resulting polymer remains no functional group having a reactivity with silica, so that the tensile strength of a vulcanizate using silica as a filler is insufficient. Furthermore, the rubber obtained by compounding silica with this polymer enhances the viscosity and green strength at an unvulcanized state and the rolling and extruding properties can be improved, but has a drawback that the permanent elongation and dynamic heat build-up are large.
In addition, Japanese Patent laid open No. 56-104,906 discloses the addition of a silane compound having at least two hydrolyzable functional groups in its molecule and represented by the following general formula: EQU SiX.sub.n Y.sub.m R.sub.4-n-m
(wherein X is a halogen atom, Y is a hydrolyzable organic group other than halogen, R is an alkyl group, an aryl group, a vinyl group or a halogenated alkyl group, n is 0 or 1 and m is an integer of 1 to 4, provided that a sum of n and m is at least 2). In this case, an alkoxy group is preferred as the hydrolyzable organic group Y other than halogen. As a most preferable silane compound, there are mentioned tetraethoxysilane, triethoxymonochlorosilane, diethoxymonochloromonomethylsilane, triethoxymonomethylsilane, trimethoxymonomethylsilane, diethoxydimethylsilane, dimethoxydimethylsilane, dimethyldiacetoxysilane, methyltriacetoxysilane, chloromethyltriethoxysilane and 3-chloropropyltriethoxysilane.
However, the hydrolyzable alkoxy group is hydrolyzed after the completion of polymerization reaction and in the steam solidification, so that it is impossible to improve the tire performances by using the resulting rubbery polymer modified with silane compound in a rubber composition containing silica as a reinforcing agent.
The invention is to solve the aforementioned conventional technical problems and to simultaneously improve the tire performances such as wear resistance, cut resistance, heat built-up and the like and economically achieve the improvement of processability, which are difficult in the conventional technique, by using a rubber composition having sufficiently high tensile strength and wear resistance even at a vulcanized state containing a white filler such as silica or the like in a tire without using a large amount of the conventional reinforcing assistant such as silane coupling agent or the like.